Common practice at present for the measurement of angular orientation in three-dimensional space is to use three-axis magnetometers to resolve orientation of an object in a magnetic field such as, for example, the magnetic field of the Earth. The measurement of angular orientation may be employed, for example, in ambulatory systems, electronic guidance and compass systems. Ambulatory systems measure three-dimensional body segment motion such as, for example, fluctuations of movements in persons with degenerative neurological disorders, measuring the level of functional impairment in persons with rheumatic and musculoskeletal diseases, assessing gait and balance deficits in the elderly and persons with neurologic disease, monitoring and documenting the progression of neurological, rheumatic and musculoskeletal diseases and disorders, developing objective outcome measures of the efficacy of rehabilitation programs, surgical interventions, and drug treatments in human diseases and disorders that affect body movement, better understanding the injury mechanisms associated with occupational, recreational and sports activities, and assessing the performance of activities of daily living, as well as occupational, recreational and sports activities in the environments in which they are typically conducted.
A variety of sensors are known in the art for measuring magnetic fields such as, for example, Hall effect sensors, proton quantum interference detectors "SQUID", fluxgate magnetometers, inductive pickup sensors and magnetoresistive sensors.
Three-axis magnetometers that can be used to estimate orientation with respect to an ambient magnetic field, such as the Earth's magnetic field, are commercially available from several companies. However, the practical use of these systems has been restricted by several significant technical limitations. First, the accuracy of these devices is dependent on the sensitivity and offset of each sensor. Since these devices attempt to directly measure the magnetic field with the magnetic sensors themselves, each sensor must maintain zero and offset values, and have low drift characteristics with temperature change. Also, most magnetic field sensors measure sums of vector components of the imposed magnetic field, which makes the determination of orientation problematic unless precise knowledge of the local magnetic field is available. When employing the Earth's magnetic field, the system software that resolves the angular orientation from the sensor data must have accurate values for the magnitude and direction of the Earth's field vector for every location where the system is used. Databases are available that provide this information for specific locations (latitude and longitude) on the Earth's surface. However, there is no easy way to determine if the Earth's magnetic field is distorted in a particular area, and significant errors will result if the database information is used in such an area.
If the Earth's magnetic field is distorted, if the magnitude and direction of the Earth's magnetic field are unknown, or if the sensor offsets are changed, then the magnetometer must be recalibrated to maintain the accuracy of the measurements. The user can recalibrate by carefully rotating the magnetometer a full revolution about each of its three axes, while monitoring the output of the sensors. The validity of this calibration procedure is predicated on the three sensors having the same sensitivity.
Because of this burden of complexity, as well as the high price for such systems, the three orthogonal sensor system, or three-axis magnetometer, has not seen much practical use except in a tightly controlled laboratory environment. Thus, there is a need in the sensor art for a sensor system that may measure angular positioning in three axes while eliminating the burden of complexity that influences the operation of existing sensor art.
Accordingly, it is an object of the present invention to provide a sensor system which overcomes the above-described disadvantages of prior devices for determining the orientation of an object in a magnetic field.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.